1. Field of the Invention
The present invention relates to a method for removing soot or the like adhered to surfaces of heat transfer tubes in a heat-exchanger of an exhaust gas economizer or the like by scattering steel balls, and a heat-exchanger provided with a steel ball scatterer.
2. Description of the Prior Art
In order to remove soot or the like adhering to surfaces of heat transfer tubes in a heat-exchanger of an exhaust gas economizer or the like, a heat-exchanger having a steel ball scattering device assembled therein has heretofore come into practical use. FIG. 1 is a general vertical cross-sectional view of one example of such a heat-exchanger in the prior art, and FIG. 2 is a perspective view, partly cut away, of the same heat-exchanger.
In these figures, reference numeral 1 designates a main body casing of a heat-exchanger, in which heat transfer tube groups 2 are disposed and steel ball scatterers 3 are provided above (upstream of) the heat transfer tube groups. Steel balls are fed to these steel ball scatterers 3 from a steel ball feeder 4. The steel balls scattered by the steel ball scatterers 3 fall while removing soot or the like adhered to the heat transfer tube groups 2. Then they are returned to the above-mentioned steel ball feeder 4 by a steel ball conveyor 5. Reference numeral 6 designates a gas inlet, and numeral 7 designates a gas outlet. The gas inlet 6 is provided at one end of the heat-exchanger main body 1 above the steel ball scatterers 3, and the gas outlet 7 is provided at one side portion of the heat-exchanger main body 1 at a level lower than that at which the heat transfer tube groups 2 are disposed.
FIG. 3 is a perspective view of one example of the steel ball scatterer 3, and in this figure, reference numeral 3a designates a steel ball feed pipe having a square cross section and numeral 3b designates a scattering plate, whose upper surface is spherical. The number of steel ball scatterers 3 disposed within the heat-exchanger is determined depending upon the projected cross-sectional area of the heat transfer tube groups and the area over which one steel ball scatterer can scatter steel balls. If the steel ball scattering area of one steel ball scatterer is broad, the number of steel ball scatterers can be small.
When removing soot or the like, adhered to surfaces of heat transfer tubes in a heat-exchanger of an exhaust gas economizer or the like, by scattering steel balls with the above-described steel ball scattering device, the rate and method of scattering the steel balls are regulated depending upon the amount of soot or the like adhered to the heat transfer tubes. More particularly, in the case where the adhered amount is great (an adhering rate is large), unless steel balls are continuously scattered at a large rate, the adhered amount of soot or the like would increase and a predetermined heat transfer performance cannot be maintained. On the other hand, in the case where the adhered amount is little, a heat transfer performance could be maintained even if the scattering rate is small or even if an intermittent scattering at long time intervals is effected.
In addition, the scattering range and scattering height of the steel ball scatterer are, in the case of the spherical surface type scatterer shown in FIG. 3, represented by the following equations: EQU scattering range x=.eta..multidot.vo.multidot.cos .THETA..multidot.t EQU scattering height y=.eta..multidot.vo.multidot..THETA..multidot.t-1/2g.multidot.t.sup.2
where
.eta.: restitution coefficient between a steel ball and the scattering plate, PA1 vo: velocity of a steel ball when it collides with the scattering plate, PA1 .THETA.: angle (with respect to the horizontal direction) of a trajectory of a steel ball flying out of the scatterer, PA1 t: time elapsed after collision, and PA1 g: acceleration by gravity.
As will be seen from these equations, a scattering range as well as a scattering height are related to the velocity (vo) of a steel ball when it collides with a scattering plate. (This collision velocity (vo) is proportional to the square root of the height from which the steel ball experiences free fall.) Accordingly, as steel balls are dropped onto a scattering plate from a higher position, the steel balls can be scattered over a broader range.
In the case of removing soot by the intermittent scattering of steel balls, the soot or the like once freed from the heat transfer tubes would scatter simultaneously with a subsequent scattering of the steel balls, and so, the concentration of soot and the like in the exhaust gas would be temporarily increased. Generally, an electric dust collector is disposed on the downstream side of a heat-exchanger, and if its dust collecting power is insufficient, soot or the like would be released into the atmospheric air, and the atmospheric air would be contaminated. Therefore, to accommodate for an abrupt increase in soot concentration in the exhaust gas as described above, it is necessary to provide an electric dust collector having a fairly large capacity, thus contributing to the overall scale of the apparatus.
In addition, in the method for removing soot by scattering steel balls as described above, since the steel balls are made to directly collide with the heat transfer tubes, the heat transfer tube fins are inevitably damaged. FIG. 4 is a longitudinal cross-sectional view of part of a finned heat transfer tube 8, and FIG. 5 is a transverse cross-sectional view of the same. With reference to these figures, a top portion of a fin 8b mounted to a pipe 8a is damaged and deformed by collision with a falling steel ball as shown at 8c in FIG. 4. The degree of damage depends upon the velocity of the steel ball, i.e., the larger the velocity, the greater the damage. The velocity at which a steel ball will collide against a fin depends upon the height from which the steel ball free falls as described above, and upon the height from which the steel ball is scattered. A broadening of the scattering range of steel balls by a steel ball scatterer is necessarily accompanied by an increase in the scattering height and thus in the damage to the fins. However, if damage to the fins is lessened by narrowing the scattering range of the steel balls, a large number of steel ball scatterers must be provided which may be practically impossible.